Post-embryonic hair cell production is being studied in a number of different systems with the aim of evaluating the feasibility of replacing damaged hair cells in humans. These studies have shown that: 1) new hair cells are normally produced in post-embryonic fish; new hair cells are produced in response to hair cell loss in salamanders and birds; 2) and new hair cells are not produced in post-embryonic mammals. Studies of the factors that account for the ability, or lack thereof, to produce new hair cells in these systems could lead to treatments to induce hair cell production in hearing impaired humans. My ongoing research deals with one of these systems, the normal post-embryonic addition of hair cells in a cichlid fish, the oscar. Post-embryonic oscars produce many new hair cells and we have identified the likely proliferating precursors that give rise to the new hair cells and neurons. At this junction, the answers to two important questions will determine the direction of research on this system. First, are the precursors to hair cells truly a stem cell population, or are they mature supporting cells that have re-entered the cell cycle? Second, are the precursors eventually depleted as the oscars age? The proposed experiments will answer these two questions.